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Chemical Formula

 
3.3  Chemical Formula
 
Definition of Chemical Formula
  • Is a representation of a chemical substance using alphabets to represent the atoms and subscript numbers to show the number of each type of atoms found in the elementary entities of the substance.
 
Example of Chemical Formula
Name of Compound Chemical Formula Number of Each Element in the Compound
Helium He One helium atom
Oxygen

\(O_2\)

 2 oxygen atoms
Water \(H_2O\) 2 hydrogen atoms and 1 oxygen atom
Ammonia \(NH_3\)

1 nitrogen atom and 3 hydrogen atoms
Sulphuric acid \(H_2SO_4\) 2 hydrogen atoms, 1 sulphur atom and 4 oxygen atoms
 
Example of the Formation of Chemical Formula
  • The chemical formula of iron(III) chloride \(FeCl_3\)
    • Iron(III) ion = \(Fe^{+3}\)
    • Chlorine ion = \(Cl^{-}\)
    • Iron, Fe has three positive charges, and Cl has one negative charge.
    • When these two types of atoms are reacted together, the charge from both atoms needs to be balanced.
    • 3 positive charge from iron needs three negative charges to make the charge equal to zero.
    • Since chlorine only has on negative charge; thus, the iron required three chlorine atom to neutralise the three positive charge.
    • Therefore the chemical formula would become, \(FeCl_3\).  
 
Formation of Iron(III) ChlorideCompound
The image shows a chemical reaction diagram for the formation of iron(III) chloride (FeCl3). It starts with iron (Fe) with a 3+ charge and chlorine (Cl) with a negative charge. The arrows indicate the combination of Fe and Cl to form FeCl3.
 
Empirical Formula
Definition of Empirical Formula
  • The chemical formula that shows the simplest ratio of the number of atoms of each element in a compound.
Steps in Determining the Empirical Formula of a Compound

The diagram shows STEPS IN DETERMINING THE EMPIRICAL FORMULA OF A COMPOUND. 1. Determine the mass of each element. 2. Determine the number of mole. 3. Simplify the number of mol using the same factor. 4. Multiply the number of mole with a factor to get an integer.
Example

 

Element C H
Mass (g) \(96\) \(18\)
Number of Moles \(\dfrac{96}{12}=8\) \(\dfrac{18}{1}=18\)
Mole Ratio \(8\) \(18\)
Simplest Mol Ratio \(\dfrac{8}{2} =4\) \(\dfrac{18}{2} = 9\)
Empirical Formula \(C_4H_9\)

 
 
Empirical Formulae of Several Substances
Water \(H_2O\)
Ammonia \(NH_3\)
Hydrazine \(NH_2\)
Propene \(CH_2\)
Benzene \(CH\)
 

 

 

 

 

 

 

The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.

Chemical Formula

 
3.3  Chemical Formula
 
Definition of Chemical Formula
  • Is a representation of a chemical substance using alphabets to represent the atoms and subscript numbers to show the number of each type of atoms found in the elementary entities of the substance.
 
Example of Chemical Formula
Name of Compound Chemical Formula Number of Each Element in the Compound
Helium He One helium atom
Oxygen

\(O_2\)

 2 oxygen atoms
Water \(H_2O\) 2 hydrogen atoms and 1 oxygen atom
Ammonia \(NH_3\)

1 nitrogen atom and 3 hydrogen atoms
Sulphuric acid \(H_2SO_4\) 2 hydrogen atoms, 1 sulphur atom and 4 oxygen atoms
 
Example of the Formation of Chemical Formula
  • The chemical formula of iron(III) chloride \(FeCl_3\)
    • Iron(III) ion = \(Fe^{+3}\)
    • Chlorine ion = \(Cl^{-}\)
    • Iron, Fe has three positive charges, and Cl has one negative charge.
    • When these two types of atoms are reacted together, the charge from both atoms needs to be balanced.
    • 3 positive charge from iron needs three negative charges to make the charge equal to zero.
    • Since chlorine only has on negative charge; thus, the iron required three chlorine atom to neutralise the three positive charge.
    • Therefore the chemical formula would become, \(FeCl_3\).  
 
Formation of Iron(III) ChlorideCompound
The image shows a chemical reaction diagram for the formation of iron(III) chloride (FeCl3). It starts with iron (Fe) with a 3+ charge and chlorine (Cl) with a negative charge. The arrows indicate the combination of Fe and Cl to form FeCl3.
 
Empirical Formula
Definition of Empirical Formula
  • The chemical formula that shows the simplest ratio of the number of atoms of each element in a compound.
Steps in Determining the Empirical Formula of a Compound

The diagram shows STEPS IN DETERMINING THE EMPIRICAL FORMULA OF A COMPOUND. 1. Determine the mass of each element. 2. Determine the number of mole. 3. Simplify the number of mol using the same factor. 4. Multiply the number of mole with a factor to get an integer.
Example

 

Element C H
Mass (g) \(96\) \(18\)
Number of Moles \(\dfrac{96}{12}=8\) \(\dfrac{18}{1}=18\)
Mole Ratio \(8\) \(18\)
Simplest Mol Ratio \(\dfrac{8}{2} =4\) \(\dfrac{18}{2} = 9\)
Empirical Formula \(C_4H_9\)

 
 
Empirical Formulae of Several Substances
Water \(H_2O\)
Ammonia \(NH_3\)
Hydrazine \(NH_2\)
Propene \(CH_2\)
Benzene \(CH\)
 

 

 

 

 

 

 

The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.
Chapter : Mole Concept, Chemical Formulae and Equations
Topic : Developing chemical formulae for compounds
Form 4 Chemistry
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